The inspection of leads of Integrated Circuit (IC) packages are extremely important to the electronics industries. IC packages such as Quad Flat Pack (QFP), Plastic Leadless Chip Carrier (PLCC), Small Outline IC (SOIC), Small Outline J-Lead (SOJ), DPAK, Small Outline Transistor (SOT), and their derivatives having leads protruding out from the IC package body. These leads are the means for connecting the internal circuits to the outside world. The integrity of these leads is crucial to provide good electrical connectivity and therefore useful application of the IC.
An Integrated Circuit (IC) package typically has a square or rectangular plastic package moulded over the IC circuitry commonly known as the "die". The size of the package may range from 4.times.4 mm square to 32.times.32 mm square. Extending from the plastic package are leads that provide electrical connectivity from the die inside the IC package to the printed circuit boards (PCB). It is important for the IC package and the leads to possess accurate and consistent mechanical dimensions because of the use of highly automated PCB assembly machine to place and solder the ICs onto the PCB. Damaged, twisted, or out-of-place leads will likely lead to improper assembling and malfunction in the PCB assembly. In particular, for high lead count IC such as the Quad Flat Pack (QFP), which has leads on all four sides of the package, the mechanical requirements are even more stringent. There are several requirements of the leads and categories of defects that have to be measured include coplanarity, lead pitch, terminal dimension, standoff, and others. Lead defects include bent leads, solder plating defects, swept leads, burr, and the like.
Several techniques and systems that include special optical and lighting arrangements for the direct and indirect viewing of leads of IC packages are available on the market. There are two major categories of IC lead inspection and measurement systems. One category uses laser-scanning approach. The other commonly used techniques include shadow casting and back illuminating the lead profiles onto imaging planes. As an example, some method uses lasers to scan the leads of an IC from the top. Another uses a back illuminated system with image doubler that increases the resolution of the image. Yet another method uses a lead inspection system to locate the leads with reference to a reference plate on which the device is mounted and a real-time reference which is used to provide a known correlation between the image pixels and linear measurement One IC inspection system includes a displacement sensor in which the upward and downward coplanarity error of each lead is measured from a level change in the output signal of the sensor. Another system determines a position of at least one lead of an electronic component using shadow casting techniques.
However, various problems exist with these prior inspection techniques. In the case of laser scanning technique, the top surface instead of the bottom surface of a lead is measured. This presents a problem because the bottom surface and geometry of a lead is more important than the top surface, particularly in relation to the electrical connectivity of the leads. The thickness of the leads will vary from lead to lead as a result of the solder plating thickness. As a consequence, measuring the top surface of the leads is not equivalent to measuring the bottom surface. This is especially true in high precision measurement in the range of several micrometers. In addition, laser-scanning techniques cannot detect burrs on lead tips, which commonly occur as a result of the trim and form process in the manufacture of IC leads. The existence of burrs on lead tips is another important factor that influences the electrical connectivity of an IC package to PCB. Therefore, there is a need for methods and systems for reliable inspection of ICs, especially for a method and methods and systems that can reliably inspect the bottom surface of the IC leads.